1. Field of the Invention
This invention relates to the manufacture of high purity anhydrous hydrogen fluoride with low levels of oxidizable impurities. More specifically but not by way of limitation, this invention deals with a manufacturing process which involves oxidizing the volatile trivalent arsenic impurity to a non-volatile arsenic pentavalent compound using alternating current in an electrolytic cell. The resulting mixture is distilled to recover high purity anhydrous hydrogen fluoride with very low levels of arsenic, water and other oxidizable impurities.
2. Description of the Related Art
Anhydrous hydrogen fluoride is formed by heating a mixture of fluorspar and sulfuric acid. The main impurities in the prepared hydrogen fluoride are flurosulfonic acid, silicon tetrafluoride, sulfur dioxide, sulfuric acid and water. These impurities are usually removed by fractional distillation, and the resulting anhydrous hydrogen fluoride has a purity of about 99.8% or better. However, the anhydrous hydrogen fluoride thus obtained still contains unacceptable quantities of undesirable impurities such as arsenic. The amount of arsenic impurity which is present in the anhydrous hydrogen fluoride depends on the amount of arsenic-bearing impurities in the fluorspar used to prepare the anhydrous hydrogen fluoride.
The anhydrous hydrogen fluoride generally contains about 20 to 600 part per million (ppm) of arsenic impurity. The presence of this impurity at these levels is undesirable for many applications. Anhydrous hydrogen fluoride is extensively used as a fluorinating agent in the production of organic and inorganic chemicals, and in the refining of metals. The presence of arsenic impurities in the anhydrous hydrogen fluoride can poison catalysts, contaminate products and cause severe equipment corrosion.
Several processes have been proposed to remove arsenic impurities from anhydrous hydrogen fluoride. Among these are processes which involve multiple fractional distillation. However, such methods are not considered to be economical for producing commercial quantities.
For example, in U.S. Pat. No. 3,687,622 the distillation of impure anhydrous hydrogen fluoride containing 1200 ppm As at very high pressures (e.g., &gt;115 psia and preferably&gt;165 psia) is disclosed wherein the As is removed overhead and purified hydrogen fluoride (e.g., &lt;3,000 ppb and preferably&lt;100 ppb As) is recovered as bottoms product. In U.S. Pat. No. 3,663,382 As impurities are removed from anhydrous hydrogen fluoride by distillation at a pressure below 25 psia with purified hydrogen fluoride being recovered as the overhead product.
Most of the processes disclosed in the art to reduce arsenic levels in anhydrous hydrogen fluoride involve oxidation of trivalent arsenic to pentavalent arsenic thereby to reduce its volatility.
For example, U.S. Pat. No. 3,166,379 discloses a method whereby high purity aqueous hydrogen fluoride is produced by treatment with an oxidizing agent, such as permanganate, persulfate, or chlorate salts together with a halogen, preferably iodine, to convert arsenic impurities to non-volatile compounds. This process is suitable for the purification of aqueous hydrogen fluoride but suffers from the drawback of possible contamination with volatile manganese compounds when anhydrous hydrogen fluoride is distilled from the aqueous solution.
A solution to this problem is claimed in U.S. Pat. No. 3,689,370 which describes a process involving the addition of an inorganic ferrous salt to the anhydrous hydrogen fluoride after the permanganate or dichromate treatment to reduce excess oxidizing agents. However, this process results in a high level of iron contamination.
The iron contamination problem is solved in U.S. Pat. No. 4,032,621 which describes a process involving the addition of a heavy-metal-free reagent, such as hydrogen peroxide, to anhydrous hydrogen fluoride after the permanganate or dichromate treatment to reduce excess oxidizing agent. This process is very sensitive and may not be convenient for continuous operation.
In East German Patent No. 62,309 (Jun. 20, 1968) the removal of arsenic from 80-90% aqueous hydrogen fluoride with 30% hydrogen peroxide (H.sub.2 O.sub.2) at 40.degree.-60.degree. C. is disclosed. The reference also teaches that arsenic removal from anhydrous hydrogen fluoride can be accomplished by continuously adding anhydrous hydrogen fluoride and hydrogen peroxide solution of suitable concentration to the reactor maintained at 80-90% hydrogen fluoride and 40.degree.-60.degree. C. The reactor contents are distilled and absorbed in water to produce a purified aqueous solution of HF. This process is limited to production of aqueous solutions of HF product and is not suitable for the manufacture of purified anhydrous hydrogen fluoride.
U.S. Pat. No. 4,083,941 claims removal of arsenic and sulfite impurities from anhydrous hydrogen fluoride by the addition of 0.7% H.sub.2 O.sub.2 and 0.6% methanol at 0.degree.-75.degree. C. or by the addition of at least 2.3% by weight of persulfuric acid (H.sub.2 SO.sub.5) based on anhydrous hydrogen fluoride. But this procedure has the disadvantage of introducing a foreign material to the process.
U.S. Pat. No. 4,491,570 claims arsenic removal from anhydrous hydrogen fluoride by treating with hydrogen chloride or a fluoride salt, or both, and then separating purified anhydrous hydrogen fluoride by distillation from the non-volatile arsenic compounds. Again, this process has the potential of introducing a foreign material into purified anhydrous hydrogen fluoride.
U.S. Pat. No. 4,668,497 discloses a process involving the addition of fluorine to oxidize impurities present in hydrogen fluoride followed by distillation. This process requires the hydrolysis of remaining fluorine after distillation.
U.S. Pat. No. 4,756,899 claims arsenic removal from anhydrous hydrogen fluoride by treating with hydrogen peroxide in the presence of a catalyst, which is comprised of molybdenum or an inorganic molybdenum compound, and a phosphate compound followed by distillation. Again, this process has the potential of introducing foreign material into the purified anhydrous hydrogen fluoride.
In a co-pending and commonly assigned U.S. patent application Ser. No. 07/487,437, a method of purifying anhydrous hydrogen fluoride using an electrolysis process is described and claimed. This method involved the use of a pair of inert electrodes immersed in anhydrous hydrogen fluoride to pass an effective amount of direct current through the anhydrous hydrogen fluoride to oxidize substantially all trivalent arsenic impurities to pentavalent arsenic. The resulting solution is then distilled and the anhydrous hydrogen fluoride with reduced levels of arsenic impurities being recovered as the distillate. The present invention is viewed as an improvement and extension of this previous method wherein it has been discovered that an effective amount of alternating current can be employed to oxidize the trivalent arsenic to the pentavalent state irregardless of the logical conclusion that reversal of polarity at the electrodes would merely result in reduction of the newly formed pentavalent state back to the trivalent arsenic. U.S. patent application Ser. No. 07/487,437 is incorporated herein by reference as further describing the electrolytic purification of anhydrous hydrogen fluoride.